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Incorrect Theories of "Lift"

Tony, did not read the link as I am hungry. That comes first.
But, I learned 4 decades ago.... lift is created by money.
Simple physics, but true.
 
I should be working for NASA as I never believed that theory. What would make anyone thing that two molecules next to each other at the leading edge would exit the wing next to each other.
 
FAA theory of lift

I have always had a problem with the way lift is explained. It generally starts with Bernoulli's theorem, and the example is a Venturi (such as the throat of a carburetor). Then we are shown a wing, with the curved surface on top and a (mostly) flat surface on the bottom. We are told that this is half of a Venturi, but not told how it works, since there is no barrier for the top edge of the airflow. That's when most of us just press the PFM button and accept it. My theory, and the way it makes the most sense to me, is that the barrier (or the other surface of the half Venturi) is formed by the column of air above the surface. We know it has weight (14.7 psi at sea level, assuming a standard day), so it exerts a force on the top of the wing, and acts as a barrier. This is my rationalization of what causes lift - the Venturi effect of the atmospheric pressure acting on the top of the wing constraining the flow over the upper surface of the wing, and thus causing a reduction in pressure on the top surface in relation to the bottom surface. This also explains (to my mind, anyway) why generated lift decreases with altitude, since the weight of the column of air above the top surface of the wing decreases as altitude increases.

Just my opinion, and worth everything you paid for it.
 
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Airflow

The airflow speeding up over the top of the wing theory has always left me a little confused too, but mainly because any explanations I have read always seem to talk about the flow of air molecules and the differences in pressure created over and under the wing. But first a disclaimer: I am in no way an expert in this so what I am suggesting here is more in the form of a request for my own understanding. But be warned, any long scientific words or calculations will probably make my eyes glaze over and my forehead end up resting on the keyboard. So, here goes?

So why is it we are taught to think in terms of air flowing over a wing? The way I look at it the air is stationary (discounting wind effect etc) and it is the aircraft/wing that is moving, effectively forcing the air molecules to accelerate from zero, upward and to a degree slightly forward of their starting point, and then I?m guessing slightly backward as the wing passes by. This seems to me like a different kind of action that would be more in keeping with the idea of changing the speed (from zero) and direction (up and forward) of air molecules to generate a lifting force.

Is it effectively the same thing as air flowing over a stationary wing? What am I missing here?

Clive Whittfield
 
"stick and rudder" says planes fly by pushing air down, which is very true. Angle of attack and dynamic pressure is all a wing cares about.

The air going faster over the top and creating less pressure on top than the bottom is true, but not really that important. More of your lift comes from having an angle of attack and dynamic pressure.
 
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To mess with your minds... A PhD student at my company showed me how our principals of magnetic induction could be explained by applying relativity to electrostatic attraction.... and getting the same result! Very cool and mind bending. Maxwell = Einstein

So, I think that various theories of lift are all equivalent in the end. You can apply Newton or Bernoulli and get the same results.
 
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"stick and rudder" says planes fly by pushing air down, which is very true. Angle of attack and dynamic pressure is all a wing cares about.

The air going faster over the top and creating less pressure on top than the bottom is true, but not really that important. More of your lift comes from having an angle of attack and dynamic pressure.

Any time someone asks me how wings make lift, I try to reframe the conversation. There's nothing magic about an airfoil that enables it to generate lift - anyone who's stuck their hand out the window of a moving car knows this. If you deflect air downward, you create lift.

The problem isn't "how to make lift," it's "how to make lift with a minimum of drag." So instead of being magic lift-generating devices, airfoils are just aerodynamically efficient air deflectors.

Disclaimer: Not an engineer. I bailed out of an ME degree after a couple years...
 
Yes, the mass of air being forced down results in an equal and opposite reaction... the force pushing the plane up. The airfoil is configured to do this with minimum drag so that the engine can keep up with the adding of the required energy back into the "system" to overcome that lost by drag.

IOW, apply enough thrust to pretty much any shape and weight of an object and it too will fly. :cool:

Bevan
 
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Yes, the mass of air being forced down results in an equal and opposite reaction... the force pushing the plane up. The airfoil is configured to do this with minimum drag so that the engine can keep up with the adding the required energy into into the "system". No? The Jato bottle mounted on the old car comes to mind. :cool:

Bevan
 
My other aeroplane is a Corby Starlet, which has fabric covering on the wings aft of the spar. If Bernoulli was correct the fabric should tend to bulge upwards when flying. It doesn't, it is definitely depressed, so how can it be a low pressure area.
I don't know who first thought of Bernoulli theorem for aircraft, but I still believe it is just the pushing down of air that does the job.
 
Finally NASA has come around to my way of thinking. I've always said, "Who says that two molecules hanging around near each other have to end up next to each other once they have gone different directions from a passing wing?" NASA's second bullet point agrees with me.
 
49clipper

My other aeroplane is a Corby Starlet, which has fabric covering on the wings aft of the spar. If Bernoulli was correct the fabric should tend to bulge upwards when flying. It doesn't, it is definitely depressed, so how can it be a low pressure area.
I don't know who first thought of Bernoulli theorem for aircraft, but I still believe it is just the pushing down of air that does the job.

Contrary to your thoughts. My Pa-16 clipper inflight photo clearly shows bulging in between the ribs in flight on top of the wing and from underneath concave fabric between rigs. Obviously low pressure on top and high pressure above. Have seen that on many aircraft. the distance between the ribs on the cub wing are far enough apart to make it obvious.
 
I don't know who first thought of Bernoulli theorem for aircraft, but I still believe it is just the pushing down of air that does the job.

The authors don't say Bernoulli's theorem is wrong, but rather the calculation of air velocity that is being used is incorrect.
Saying that physics 'explains' lift is fundamentally incorrect. Nature is what it is. Physics tells you how to calculate the observation. It is only an explanation to the extent that one can relate to something similar in his common experience. In this case, Bernoulli's theorem (using the correct velocities) or Newton's law (every action has an equal and opposite reaction) applied to the down wash will give the same answer.
 
In this case, Bernoulli's theorem (using the correct velocities) or Newton's law (every action has an equal and opposite reaction) applied to the down wash will give the same answer.

Or "trump's law" which says money makes everything work.

Bevan
 
Contrary to your thoughts. My Pa-16 clipper inflight photo clearly shows bulging in between the ribs in flight on top of the wing and from underneath concave fabric between rigs. Obviously low pressure on top and high pressure above. Have seen that on many aircraft. the distance between the ribs on the cub wing are far enough apart to make it obvious.

... I also have two fabric covered aircraft that exhibit this same behavior and have witnessed in many times in others. Thanks, Allan..:D
 
A friend of mine works for Boeing, and helped design the wing on the 787. He made me promise never to tell anyone that:"We have no idea what really makes a wing fly". Take a look at this photo of a wing that struck a turkey vulture in flight.

http://www.vansairforce.com/community/showthread.php?t=94981&highlight=Turkey+vulture

It's hard to imagine there could be an undisturbed flow of faster air traveling over the top of that wing, and therefore hard to believe Bernoulli's principal contributes to a significant portion of lift. Despite the damage, the OP stated that the airplane flew quite well after the strike.

I think it's Keebler Elves.... :D
 
the first thing wrong with that picture is that it shows the top and bottom flows dividing at 0% chord. That's not what happens. That point, called the stagnation point, moves well down on the airfoil as AOA increases. That's why those little tab switches work on cessnas to give you the stall warning horn. The distance traveled on the upper surface is much longer, even on a flat plate, because of this movement of stagnation.

If you put lots of pressure ports on the top and bottom of an airfoil in a wind tunnel and measure all the pressures and add them up and multiply by the areas that they represent you will get something pretty close to the lift being measured by the wind tunnel balance. I have done it often. When we test scale models of biz jets and passenger jets we put pressure taps on the wings to check the pressures vs our computer models. The pressures from either the computer model or the wind tunnel pressure ports can be added up to calculate lift. We usually find we are very close in our estimates of lift when we fly the airplane. The computations tend to fall down right around stall, because the prediction of flow separation is more art than science and once the flow separates in one place it influences everything around it. This can also be difficult to get right even in a wind tunnel, due to scaling effects. But in the linear range, before stall, lift calculated by adding up (or integrating) pressures is pretty accurate.

And if you were to measure the change in momentum of the air being pushed down by the wing you would also get the same number if you could measure it accurately. The advantage of using pressures is that you can calculate the forces on any part of the airplane and thus work out the moments as well as the over all force. Measureing the momentum just gives you the overall lift. Not useful for predicting the loads on different parts of the airframe.

The relationship of velocity vs pressure, as predicted by Bernoulli, who did this long before man had built wings, is correct.
 
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Listen while you work...

This podcast (when you get to the good stuff at the end) begins to hint at the complexity of lift.

Note that this is perhaps not the best of Marcus' podcasts. You can find others on flight (interviews with SR71 and U2 pilots, for example) on http://omegataupodcast.net.
 
Its just like holding your had outside a car window and twisting it. Put a model aircraft engine on a rectangular balsa wood door and it will fly. The fancy curving just makes it more efficient.

:D:D:confused::confused:
 
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